The invention relates to systems and methods for detecting the presence, or determining the location or the size, of an object in specific zones or areas. Such systems are typically used to restrict personnel access to specific areas of a building, to stop or deactivate dangerous machines when workers approach them, or to locate and count objects on a conveyor belt. It also relates to systems for detecting changes in the conductivity, e.g. due to changes in the dielectric constant, of an object. Such systems can be used to e.g. detect changes in the humidity or viscosity of a medium or objects within a predefined space.
A known class of detector systems typically use a combination of light emitting sources and light sensing devices, where a light beam between a source and a sensor is interrupted by the object to be detected when it crosses the line of sight between the source and the sensor. Such systems have several limitations, especially when used in harsh environments. First, the object has to pass through a specific line, or a limited number of lines, in space in order to be detected. Depending on the application, this requirement may lead to objects not being detected by the system. Furthermore, both light sources and light sensors may be subjected to dirt, moisture, mechanical damage or chemicals, which may alter or obscure the light path whereby the object-detecting capability of the system decreases or ceases completely. Mechanical forces, e.g. from a water spray, acting on the detector system during cleaning or rinsing of the system or its surroundings, may change the alignment of source and sensor with similarly negative effects. In a typical large slaughterhouse, for instance, there may be several thousand of such optical detector systems installed for detecting, locating and counting meat cuts on various forms of conveying and/or processing apparatus, e.g. conveyor belts or conveyor rails. The apparatus must be kept clean at all times in order to avoid contamination of the meat, wherefore the apparatus, and consequently the detector systems, are frequently rinsed with hot water or steam under high pressure. The temperature changes induced thereby put a stress on the light windows of detector systems, which causes them to deteriorate or crack, which again may cause the detector systems to fail. Consequently, a large installation in a slaughterhouse requires a large amount of maintenance and repair work, which again raises the cost of running the facility. Also possible stand-stills of machinery due to defective detector systems can be very expensive, especially since meat cuts may be declassified and hence lose value due to stand-stills.
Another class of known object sensing systems reacts to changes in the capacitance between two electrodes—or between a single electrode and ground—or between a single electrode and circuit ground, the change being induced by an object moving into the zone between the electrodes or between the electrode and ground/circuit ground. The presence of the object is detected by comparing the measured capacitance to a predetermined threshold. Since electrodes can be made of stainless steel or other metallic materials, which can easily withstand the harsh environment in e.g. a slaughterhouse, it would be desirable to use this kind of detector system instead of the more fragile optical systems described above. Furthermore, such systems can easily detect a change of the conductivity of a non-moving object in the zone or area. However, a typical limitation with this kind of system is that persons or moving objects in the vicinity, which are not meant to be detected by the system, may alter the capacitance and thus either cause false alarms or mask capacitance changes caused by objects that should be detected or measured, thereby causing the system to fail. This is especially true in an environment where workers perform their work close to the conveyor belts where the detector systems are in use. Furthermore, the known systems are not precise enough to detect each of a series of objects on e.g. a conveyor belt, when these objects are close to each other. The precision of known systems is typically related to their detection range, so that systems working within a range of a few centimeters may be very precise, whereas systems with larger detection ranges are less precise. In order to enable a practical detection and/or measurement of objects like meat cuts on a conveyor belt, the detection/measurement range should be at least one half meter, preferably more, and at the same time a precision of only a few centimeters or less is desired.
The output of the detector system on a conveyor belt is typically used to detect the precise location of e.g. meat cuts, so that dedicated machines may perform controlled actions with the objects, such as grasping the objects or attaching labels to them. It would therefore be desirable to have a detector system, which can determine the exact location of the detected objects. The European Patent EP 0 850 385 discloses a method for determining the distance between two objects, which are coupled capacitively to each other, by inducing an electrical voltage signal to one of the objects and measuring the signal level on the other object. The therein disclosed method could be used to enable the detector system to determine the location of the objects, but it suffers partly from the same limitations as the capacitance based systems described above. It furthermore depends on the possibility of inducing a well known voltage level to, or measuring a level accurately on, one of the objects, which is typically difficult to obtain, especially when the objects on a conveyor belt are of at least slightly different sizes, like meat cuts typically are.